Imaging devices using image pickup devices such as CCD's (Charge Coupled Devices) and CMOS's (Complementary Metal Oxide Semiconductors) are well known. In such imaging equipment, an image is formed on the image pickup device and electrical signals from the image detector are processed in order to form a viewable image. Recently, along with advancements in miniaturizing the imaging equipment, the entire imaging system has become much smaller.
Video cameras and digital still cameras (hereinafter referred to simply as digital cameras) for image input to cell phones have recently been significantly reduced in size. Imaging lenses used for the small imaging systems used in these devices are conventionally made of a single lens element in view of the small size and portability required.
While imaging devices have been developed to be smaller in recent years, they also have been developed with increasing numbers of pixels in the images. Consequently, improved optical properties have been demanded for the imaging lenses used in these imaging devices. Imaging lenses with only one or two lens elements do not provide adequate optical properties, including correction of aberrations, in the images produced by these devices.
In order to provide for improved correction of aberrations as needed in imaging devices having a large number of pixels, imaging lenses with a larger number of lens elements, namely with three lens elements have been developed. For example, Japanese Laid-Open Patent Application 2002-221659, Japanese Laid Open Patent Application 2002-517773, and U.S. Pat. No. 6,441,971 disclose such three-element imaging lenses.
However, although increasing the number of lens elements enables the image forming properties of the imaging lens to be improved, the overall length of the imaging lens tends to become longer compared to an imaging lens having only two lens elements. Accordingly, the lenses described in the above-mentioned Japanese Laid-Open Patent Application 2002-221659 and in U.S. Pat. No. 6,441,971 do not fully satisfy the requirement for compactness. Consequently, further improvements in imaging lenses are needed in order to obtain the small size and portability desired by the marketplace.
In addition, in imaging lenses for imaging devices as described above, various optical performance characteristics may be advantageous based on the characteristics of the imaging devices used. For example, in general, when image pickup devices such as CCD's are used, due to their characteristics, it is desirable that the light rays strike the image surface nearly parallel to the surface normal, i.e. at a small angle of an incidence. Thus, it is desirable that the imaging lens be arranged to be telecentric on the image side. Therefore, for optimum imaging performance, the properties of the imaging lenses need to match the characteristics of other parts of the imaging devices.